The present invention generally relates to television cameras having an optical filter, and more particularly to a television camera having an optical filter which is capable of preventing the generation of color beat which is easily generated with respect to two directions, one direction being the scanning direction of an electron beam within the pickup tube and the other direction being perpendicular to the scanning direction, when a color separation is carried out by using the vertical correlation of output color signals in mutually adjacent scanning lines, and is also capable of preventing deterioration in the resolution.
In a television camera which uses a single pickup tube, a color separating optical system including a color stripe filter is employed. However, in such an optical system, beat is introduced between the high-frequency component of the luminance signal and the chrominance signal. Accordingly, in order to reduce the high-frequency component of the luminance signal so as to reduce the above beat, an optical lowpass filter is generally employed. Normally, such an optical lowpass filter has an attenuation pole in the filter characteristic.
In the television camera of the type which carries out the color separation by use of the vertical correlation in the picture, the chrominance signal is separated by delaying the output signal related to an N-th (N is an integer) scanning line and the output signal related to a (N+1)-th scanning line by one horizontal scanning period, or by shifting the phase by .pi./2, and carrying out series of addition, subtraction, and detection. For example, this type of a television camera employs a pickup tube which comprises an intercrossing type filter, or employs a solid state image pickup element which comprises a filter having a Bayer arrangement. In a television camera of this type, a spurious signal is generated due to the high-frequency component of the luminance signal, and color beat is generated in a first direction (generally, in the horizontal direction) which is the same as the scanning direction of the electron beam within the pickup tube and in a direction (generally, in the vertical direction) perpendicular to the first direction. The color beat with respect to the vertical direction is generated when the vertical correlation cannot be used. When scanning the vertical end of the picture, the output signal related to the N-th scanning line and the output signal of the (N+1)-th scanning line become completely unrelated to each other, and there is no vertical correlation. For example, if the N-th scanning line scans white and the (N+1)-th scanning line scans black, the desired primary colors of red (R) and blue (B) cannot be obtained, and the three primary colors of red (R), green (G), and blue (B) become unbalanced. When such an unbalance occurs, unwanted color will appear on the screen of a monitoring television receiver. Generally, an optical filter which acts in the horizontal and vertical directions, that is, in two dimensions, is inserted in the optical system so as to eliminate the above color beat.
Conventionally, as an optical filter of the type described above, there was an optical filter comprising first, second, and third separation crystal plates which are stacked as will be described later on. The first separation crystal plate was designed to separate the light beam in the scanning direction, and the second separation crystal plate was designed to separate the light beam in a direction which is rotated counterclockwise with respect to the axis of the scanning direction by 45.degree.. The third separation crystal plate was designed to separate the light beam in a direction which is rotated clockwise with respect to the above axis of the scanning direction by 45.degree..
According to the conventional optical filter described above, a single incident light beam is separated in two directions, that is, in the horizontal and vertical directions. In other words, the optical filter acts in two dimensions, and the light beam separated in the horizontal direction and the light beam separated in the vertical direction are each separated into two rays. Four rays are thus obtained from the single incident light beam. However, as will be described later on, with respect to the horizontal direction (scanning direction), it is impossible to prevent the generation of color beat in a frequency band in the vicinity of a frequency which is twice the carrier frequency. Moreover, with respect to the vertical direction (direction perpendicular to the scanning direction), it is possible to prevent the generation of color beat, however, the resolution becomes poor because of the excess effect of preventing the generation of the color beat. These undesirable phenomenons are introduced because the conventional optical filter only acts in two dimensions. As a result, there was a disadvantage in that a pattern including fine stripes could not be picked up by the television camera employing the conventional optical filter.